The present invention relates to a signal multiplexing system employing spread spectrum, particularly to that using chirp signals as spread spectrum codes enabling the multiplexing of signals by time-shifting a plurality of signals.
Description of the Related Art
As to a conventional signal multiplexing method using spread spectrum, there has been disclosed in Japanese Patent Laid-Open 7-50649 a method where data codes are directly spread through the use of spread codes.
One example of configuration concerning the conventional signal multiplexing system will be given with reference to FIG. 1. In this figure, a multiplex number is set to 2, and a symbol interval of a data code is set to Ts. A transmission code is generated by a data code generator 202. This transmission code is to be convoluted at a spreader 204 by a spread code outputted from a spread code generator 205. On the other hand, an output signal from a data code generator 201 is also spread due to being convoluted by the spread code outputted from the spread code generator 205. In this case, however, if the output signal from the data code generator 201 is spread as it is, it would be difficult to differentiate it from the output signal of the spreader 204 which leads to an impossibility of separation of the signal at a receiving side. Accordingly, a spread code, which is substantially the output signal of the spread code generator 205 being delayed by an interval of Ts/2 at a delay circuit 206, is arranged to be inputted to a spreader 203. The output signals from the spreader 203 and the spreader 204 are added together at an adder 207 to be outputted as a single signal. Then the outputted signal is modulated and altered at a modulator 208 so as to become of a signal form suitable for easy transmission.
Meanwhile, on the receiving side, the received signal is inputted to a despread filter 210. The operation at the despread filter 210 is realized by executing correlation operation with the received signal and a code identical to that generated from the spread code generator 205. Output signals of the despread filter 210 are two separated signals each of which being shifted by an interval of Ts/2. Therefore, one signal is to be sampled by a sampling device 212, and the other signal is once delayed by an interval of Ts/2 at a delay circuit 211 to be sampled by a sampling device 213 which is substantially the same as the sampling device 212. Thus, as being described above, due to having signals being multiplexed by shifting the timing of the spread codes, higher-speed signal transmission is made possible within the same frequency bandwidth.
The conventional system being described above has the following problems. Generally, the signal multiplexing method adopts a direct spread spectrum method where spread codes are applied. According to the conventional method, the chip rate of the spread code is more than 10 times faster than the symbol rate of the data code which leads to a considerable acceleration of an operation speed of the spreader. The high-speed operation of the spreader might give rise to problems concerning the operation speed and power dissipation of the elements. The reason for these particular problems is that for a portable terminal which is designed to use batteries as its power source, it is difficult to adopt a communication method where power dissipation is large.
Furthermore, while there are multipath distortion in the transmission channel, the signal multiplexing method employing the direct spread spectrum can not ignore the correlation distortion among the spread spectrum codes which leads to difficulty in transmitting.